Emission control devices, such as catalytic converters, can reduce emissions generated by engine combustion. However, the effectiveness of such emission control devices varies with operating temperature. For example, the efficiency of an emission control device may be much greater at higher temperatures than it is at lower temperature. Typically, a “light-off” temperature is used to signify a certain temperature above which a prescribed efficiency is achieved.
One approach to provide rapid catalyst heating. advantageously uses different ignition timing values for two cylinder groups, as described in U.S. Pat. No. 6,568,177.
However, the inventors herein have recognized that if different cylinder groups having different ignition timing are operated with the same amount of airflow, the engine may not achieve as much benefit as originally thought. For example, the optimal air amount for the cylinder with more retarded timing might be lower than the air flow desired for a cylinder operating with little or no ignition timing retard to provide proper cranking operation. Conversely, the optimal air amount for the for the cylinder with more retarded timing might be higher than the air flow desired for the cylinder operating with little or no ignition timing retard if there is degraded vibration experienced, such as if the retarded timing is between −10 and −40 BTDC.
At least some of these disadvantages can be overcome by a method for controlling an engine having at least a first and second cylinder. The method comprises operating the first cylinder with a first ignition timing and a first amount of fresh air flow; and operating the second cylinder with a second ignition timing, more retarded than said first ignition timing, and a second amount of fresh air flow adjusted by an actuator to be different from said first amount of fresh air flow.
In this way, it is possible to obtain the advantageous operation of rapid catalyst heating by operating one cylinder with more advanced ignition timing to generate torque to rotate the engine at a higher load (and thus with greater heat flux) and the other cylinder with more retarded timing and low torque output with high exhaust gas temperature, while at the same time improving performance in both cylinders by providing the appropriate amount of cylinder air charge. Further advantages are that by providing the appropriate amount of cylinder air charge, NVH can be reduced in some conditions, and engine torque output of the engine can be increased in other conditions.